In Garry's Mod, one of the more popular things to make is an automated turret. As gmod is based off an FPS engine (from Half-Life 2), combat is still a major focus in many multiplayer games of gmod.

Unfortunately, making a turret isn't as easy as telling a gun to point at someone and fire. Everything still follows the simulated rules of physics, and so to make a turret, you'll need to mount various parts on axes and use thrusters/motors to get those parts to spin.

Then, once you get them to spin, calculating the proper amount of spin to apply to get the turret to face your target is another problem. All objects have inertia, so you need to counter that out while having your turret spin toward your target.

Once you get your turret working properly, and calibrated so that it can hit its target, it's fun to throw all sorts of things at it to shoot, such as exploding barrels, Manhacks (small, flying robots with rotating blades), or enemy players.